A workpiece in the shape of a right circular cylinder such as a solid or hollow shaft can be metallurgically heat treated (hardened) to withstand forces that are applied to the workpiece in the intended application. For example the workpiece may be automotive components of various cylindrical shapes that are metallurgically hardened for use in motor vehicle powertrains.
More complex workpieces are formed by combining multiple cylindrical components having different diameters, fillets, shoulders, holes and other geometrical irregularities. Examples of such complex geometries are illustrated in FIG. 5.28 (right side figure) and FIG. 5.36 of the Handbook of Induction Heating (Valery Rudnev et al., 2003, Marcel Dekker, Inc., New York, N.Y.). FIG. 1(a) illustrates another example of a complex workpiece. In general these complex workpieces can be characterized as having an at least partially cylindrical component with its central axis coincident with the central axis of a circular component and connected at one end to the circular component with a diameter larger than the diameter of the at least partially cylindrical component, and for convenience, such workpieces are referred to herein as “complex workpieces.” For example, for complex workpiece 90 shown in FIG. 1(a), the workpiece component within dashed box 90a is the at least partially cylindrical component and the workpiece component within dashed box 90b is the circular component with a diameter larger than the diameter of the at least partially cylindrical component, and these two workpiece regions are oriented so that the at least partially cylindrical component 90a has its central axis CL coincident with the central axis of circular component 90b and connected at one end to the circular component 90b with an outside diameter d2 larger than the outside diameter d1 of the at least partially cylindrical component 90a. 
Electric induction heating is used in a variety of heat treatment processes, such as annealing, normalizing, surface (case) hardening, through hardening, tempering and stress relieving. One of the most popular applications of induction heat treatment is the hardening of steels, cast irons and powder metallurgy components. In some cases heat treatment of the entire workpiece is required; however in other cases it is only necessary to heat treat selected regions of the workpiece.
A typical induction hardening process involves heating the workpiece or the region of the workpiece required to be strengthened up to the austenitizing temperature; holding (if required) the workpiece or region at austenitizing temperature for a sufficient period of time to complete austenitization; and then rapidly cooling the workpiece or region to below the temperature where a desirable martensitic structure starts to form. Rapid cooling or quenching allows replacement of the diffusion-dependent transformation process by a shear-type transformation creating a much harder constituent called martensite. Martensite can be formed and hardening may be done either on the surface of the workpiece or region, or throughout the entire cross section of the workpiece or region. Workpieces are induction hardened for different reasons. For example hardening may be done to increase torsional strength and/or torsional fatigue life, to improve bending strength and/or bending fatigue life, or to improve wear resistance or contact strength.
Various types of heating inductors can be utilized to induction harden a cylindrical or complex workpiece. Since induction heating of a workpiece is dependent upon magnetic flux coupling with regions of the workpiece to induce eddy current heating in the workpiece, a uniform inductive heat treatment within complex geometry areas, such as fillets between adjacent cylindrical components, is difficult to achieve with typical induction coil arrangements. The inductive heating process is further complicated by the fact that generally heat penetration into the interior of the workpiece is a combination of both inductive eddy current heating inwardly, and then further conductive inward heat transfer from the eddy current regions (controlled by the depth of induced current penetration) towards the central region of the workpiece, which conductive heating process is known in the art as heat “soaking”
An inductor's configuration depends upon specific parameters of the application that include the geometry of the workpiece; the composition of the heated material; the available space for inductor installation; the heating mode (for example, scanning, single-shot, progressive or static heating mode); the workpiece production rate; the required heating pattern; and the details of the workpiece handling (that is, how the workpiece is loaded and unloaded).
Inductors for induction hardening are typically fabricated from copper or copper alloys because of copper's high electrical and thermal conductivities, its inherent corrosion resistance and superior cold and hot workability.
Channel-type (also known as single-shot or slot) inductors are one type of inductor that are most suitable for through and surface hardening of cylindrical and complex workpieces. With the channel inductor neither the workpiece nor the induction coil moves relative to each other except for possible rotation of the workpiece. Channel inductors can be single-turn or multi-turn inductors. Multi-turn channel inductors are typically applied for through heating of the ends of billets or bars prior to hot forming, for example, in an article forging process. Single-turn channel inductors are typically used for induction hardening cylindrical or complex components that are representatively shown in FIG. 5.28 (right side figure) and FIG. 5.36 in the Handbook of Induction Heating. Typical applications for single-turn channel inductors are hardening of carbon steel shafts such as output shafts, flanged shafts, yoke shafts, intermediate shafts and drive shafts.
A single-turn channel inductor consists of two longitudinal legs and two crossover segments (also known as bridges or horseshoe-style half loops). Crossover segments do not encircle the entire circumference of the workpiece to be heat treated but only a portion that is typically half of the circumference. When longitudinal regions of the workpiece are required to be heated, induced eddy currents primarily flow along the length of the workpiece. An exception would be the crossover segments of the channel inductor where the flow of eddy current is half circumferential. As an example FIG. 5.33 in the Handbook of Induction Heating shows a channel inductor used for induction hardening of axle shafts. Instantaneous electrical alternating current in each of the two longitudinal legs and each of the two crossover segments are in opposite directions with respect to each other.
The length of the heated region can be controlled by fabricating channel inductors with longitudinal leg sections of different lengths. FIG. 1(b) shows one example of a prior art single-turn channel inductor 100. First (upper) crossover section 102 comprises crossover half-sections 102a and 102a; longitudinal leg sections 104a and 104b and second (lower) crossover section 106. Complex workpiece 90 is inserted into single-turn saddle inductor 100 as shown in FIG. 1(c). Crossover half-sections 102a and 102a′ (FIG. 1(b)) are electrically isolated from each other, for example by dielectric slot 112 so that crossover half-sections 102a and 102a′ can be connected to the outputs of alternating current power source 114. Since the crossover sections and the longitudinal leg sections of inductor 100 only partially surround the circumference of complex workpiece 90 the workpiece is rotated about its central axis CL while loaded in the heat treatment position shown in FIG. 1(c).
Channel inductor 100 in FIG. 1(b) and FIG. 1(c) is oriented in the vertical direction for single-shot workpiece loading and removal either in the vertical or horizontal direction.
Longitudinal leg sections of a single-turn channel inductor may be profiled by relief shaping selected regions of the longitudinal legs to accommodate specific geometrical features of the heat-treated workpiece, such as changes in diameter of the workpiece. Similarly one or both crossover sections of a single-turn channel inductor can be profiled or curved for generating required magnetic field coupling with the appropriate regions of the workpiece to achieve required temperature profiles. Fabricating required section(s) of the channel inductor with narrower heating surfaces facing the workpiece can increase density of the induced power in desirable region(s).
FIG. 2(a) through FIG. 2(c) illustrate three typical examples of profiled crossover segments of prior art single-shot channel inductors near a fillet region.
FIG. 2(a) shows a lower-half crossover section 106′ of a single-shot one-turn prior art channel inductor heating apparatus for heat treating a solid complex workpiece 92. Only the right half of lower crossover section 106′ (similar to cross over section 106 in FIG. 1(b)) of a vertically oriented channel inductor is shown in FIG. 2(a) with internal cooling passage 106a′ for flow of an inductor cooling medium. Separate quench apparatus 116 is provided in this example for quenching when the workpiece achieves required thermal conditions after being heated in the channel inductor. Alternate quenching methods include quenching after the workpiece has been heated and unloaded from the channel inductor. Axis of vertical symmetry CL is indicated for the core of the solid cylindrical component 92a of complex workpiece 92. Thus for complex workpiece 92 the at least partially cylindrical component is solid shaft cylindrical component 92a and the circular component with a diameter larger than the diameter of the at least partially cylindrical component is component 92b (with cross hatching in opposing direction of the crosshatching for component 92a). Thus the at least partially cylindrical component 92a has its central axis coincident with the central axis of circular component 92b and is connected at one end to circular component 92b with a diameter larger than the diameter of the at least partially cylindrical component 92a as shown in FIG. 2(a). Outside diameter 92c and fillet region 92d of complex workpiece 92 are included in the regions for induction hardening and are shown as stippled regions. Outside diameter 92c will be heated due to induced eddy current generated by electrical current flowing in the longitudinal leg sections 104a and 104b (not shown in FIG. 2(a)) of the channel inductor. Induced heating in fillet region 92d is primarily generated by channel inductor current flowing in lower crossover section 106′ of the channel inductor.
FIG. 2(b) shows lower crossover section 106″ (in right-half view only) of a single-shot one-turn prior art channel inductor heating apparatus for heat treating hollow complex workpiece 90, which is the workpiece also shown in FIG. 1(a). Only half of lower crossover section 106″ (similar to cross over section 106 in FIG. 1(b)) of a vertically oriented channel inductor is shown in FIG. 2(b) with internal cooling passage 106a″ for flow of an inductor cooling medium. Separate quench apparatus is not shown in FIG. 2(b). Axis of vertical symmetry CL is indicated for the core of the hollow cylindrical component (90a and 90c) of complex workpiece 90 with the hollow interior core region shown without cross hatching. Thus for complex workpiece 90, as also described above relative to FIG. 1(a), the at least partially cylindrical component is hollow cylindrical component 90a and the circular component with a diameter larger than the diameter of the at least partially cylindrical component is designated component 90b in the figure so that the at least partially cylindrical component 90a has its central axis coincident with the central axis of circular component 90b and is connected at one end to circular component 90b with a diameter larger than the diameter of the at least partially cylindrical component 90a as shown in FIG. 2(b).
When a workpiece has hardening regions that include fillets as in FIG. 2(a) and FIG. 2(b) it is often necessary to substantially increase the induced heat intensity in the fillet region since the fillet region has a substantially greater mass of metal to heat. Additionally there is an appreciably larger workpiece mass in the proximity of the heated fillet and behind the region to be hardened that develop a substantial “cold” sink effect that draws heat from the heated fillet due to thermal conductivity. Therefore cooling effect of the cold sink effect must be compensated for by inducing additional heating energy in the fillet area. Required energy surplus is often achieved by narrowing the current carrying face of the appropriate section of the channel inductor to increase the induced power density within the appropriate regions. For example if the current carrying portion of the heating face of the inductor section is decreased by half then there will be corresponding increase in the inductor section's current density as well as the density of the eddy current induced within the respective workpiece region. According to the Joule effect if the density of induced eddy current doubles then the induced power density increases four times.
For the arrangements in both FIG. 2(a) and, in particular FIG. 2(b), the heating face of the inductor in the crossover region that faces the fillet region has been profiled to concentrate an induced eddy current and heat generation within the fillet region.
FIG. 2(c) shows a detail view of a lower-half crossover section 106′″ of an alternative prior art single-shot one-turn channel inductor heating apparatus where magnetic flux concentrators 80a and 80b are provided in addition to crossover section inductor profiling to provide further concentration of heating energy in the fillet region 94c of complex workpiece 94. Localized current density of an inductor can be increased appreciably when magnetic flux concentrators are utilized.
Magnetic flux concentrators (also called flux intensifiers, flux controllers, shunts, diverters, or magnetic cores) affect the electromagnetic coupling between the workpiece and the magnetic field of the channel inductor. There are several traditional functions of magnetic flux concentrators in induction hardening: (a) providing a selective heating of certain areas of the workpiece; (b) improving the electrical efficiency of the inductor; (c) and acting as an electromagnetic shield and preventing undesirable heating of adjacent areas. Flux concentrators are made from high-permeability soft-magnetic materials having low electrical conductivity. The soft-magnetic nature of flux concentrators means that they are magnetic only when an external magnetic field is applied. Upon being exposed to an alternating current magnetic field, these materials can change their magnetization rapidly without much friction. Narrow magnetic hysteresis loops of small area are typical for these materials. Concentrators provide a path of low magnetic reluctance and facilitate the concentration of flux lines in desired regions. If a magnetic flux concentrator is introduced into the inductor field, it will provide a low-reluctance path for the magnetic flux, reducing stray flux and concentrating the imaginary flux lines of magnetic field. Without a flux concentrator, the magnetic field would spread around the inductor and link with the electrically conductive surroundings (e.g., auxiliary equipment, metal support, tools, fixtures, workpiece regions that are not desirable to be heated, for example). The concentrator forms the magnetic path to guide the inductor's magnetic field in desired areas. The above-mentioned factors have potentially positive effects on induced heating selective regions. However localized current densities in certain regions of the inductor can be substantially increased and potentially causing localized inductor overheating, and/or hastening the onset of inductor stress cracking (by work hardening of the inductor, for example).
One of the main drawbacks of a conventional single-turn channel inductor is its short life. The requirement for producing sufficient heat generation in selected regions of the workpiece such as fillet regions results in the necessity of having an appreciably narrow inductor heating face in combination with using magnetic flux concentrators, which is associated with excessive coil current density and premature failure of the heating inductor. Premature inductor failure (cracking, stress-corrosion or stress fatigue) typically occurs in the region of highest current density and usually takes place in the crossover section 106 of a single-turn channel inductor that provides heating of fillets. Crossover sections also experience an inductor flexing due to the presence of electromagnetic forces. Therefore in order to increase the life of hardening inductors attempts should be taken to reduce current densities in that region.
Another drawback of conventional single-turn channel inductors is associated with an excessive process sensitivity that negatively affects quality and heating repeatability of hardened components. Excessive sensitivity is associated with an electromagnetic proximity effect. If the positioning of the workpiece inside of the channel inductor changes (for example, by wear of bearings associated with apparatus for rotating the workpiece within the inductor, incorrect loading of the workpiece in the inductor) then there will be an immediate variation of the heating intensity particularly within the fillet region. This typically results in a temperature deficit and reduced hardness depth associated with it.
One object of the present invention is to provide an improved inductor for single-shot induction heating of complex workpieces where an at least partially cylindrical component with its central axis coincident with the central axis of a circular component and connected at one end to the circular component with a diameter larger than the diameter of the at least partially cylindrical component with increased inductor life, improved robustness and reduced heating sensitivity to the workpiece positioning within the inductor.
The above and other aspects of the invention are set forth in this specification and the appended claims.